(1) Field of the Invention
The present invention relates to a substantially linear, high-molecular-weight, novel novolak type substituted phenolic resin and a process for the preparation thereof. More particularly, the present invention relates to a substantially linear, high-molecular-weight, novel novolak type phenolic resin which provides composition excellent in the heat resistance and mechanical properties when incorporated in various setting resins and which can be used in various fields.
(2) Description of the Prior Art
Ordinarily, novolak type phenolic resins are prepared by condensing a phenol with an aldehyde in the presence of an acid catalyst, and it is known that the obtained resins have a structure in which phenol units are connected through methylene groups bonded at the 2,6- or 2,4-positions thereof and in which methylol groups are not contained or if contained, the amounts of the methylol groups are very small. Novolak type phenolic resins prepared according to this known process ordinarily have a number average molecular weight of 250 to 800 and the number average molecular weight is about 1000 at highest, and also the melting points of these novolak type phenolic resins are low. Furthermore, it is known that a novolak type substituted phenolic resin having a structure similar to that of the above-mentioned novolak type phenolic resin is obtained by condensing a substituted phenol having an alkyl group or halogen atom at the para- or ortho-position of the phenol nucleus with an aldehyde in the presence of an acid catalyst. However, the number average molecular weight of this phenolic resin is similarly low and ordinarily in the range of from 250 to 800, and is 1200 at highest. Accordingly, the melting point of this phenolic resin is low. Therefore, even if this novolak type substituted resin is incorporated into other various setting resins optionally with a filler or additive and the resulting compositions are cured, there cannot be obtained cured resin compositions excellent in the heat resistance and mechanical properties, as in the case of the above-mentioned novolak phenolic resin.
As is seen from the above description, the number average molecular weights of novolak type phenol-formaldehyde resins prepared according to the conventional processes are very low and are ordinarily in the range of from 250 to 800 and 1200 at highest. It was reported that when such novolak type phenol-formaldehyde having a low number average molecular weight is fractionated, a novolak type phenol-formaldehyde resin having a molecular weight of about 3000 to about 10000 is obtained though the content is very low [see Lectures on Plastic Materials, Volume 15, Phenolic Resins, pages 14-24, written by Shinichi Murayama and published by Nikkan Kogyo Shinbunsha and J. J. Gardikes and F. M. Konrad, Am. Chem. Soc., Div. Org. Coating and Plastic Chemistry, 26, No. 1, 131-137 (1966)]. However, since a high-molecular-weight novolak type phenol-formaldehyde resin obtained by fractionation has a narrow molecular weight distribution and a gelation product is readily formed by partial crosslinking owing to the trifunctional characteristic of the phenol and is contained in the obtained resin, even if such phenolic resin is incorporated in a setting resin, it is impossible to sufficiently improve the heat resistance and mechanical properties in the resulting resin composition. Furthermore, the process for preparing such high-molecular-weight novolak type phenol-aldehyde resin, disclosed in the above references, that is, the process comprising fractionating a novolak type phenol-formaldehyde resin having a low number average molecular weight to obtain a high-molecular-weight novolak phenol-formaldehyde resin contained in a minute amount, is not advantageous from the industrial viewpoint.
Attempts have been made to obtain a high-molecular-weight novolak type substituted phenolic resin by polycondensing a bifunctional alkyl phenol with an o-alkyl or p-alkyl phenol with an aldehyde in the presence of an acid catalyst. However, the number average molecular weight of the so obtained novolak type alkyl-substituted phenolic resin is up to 1200, and a novolak type alkyl-substituted phenolic resin having a sufficiently high number average molecular weight has not been obtained [see, for example, F. S. Granger, Industrial and Engineering Chemistry, 29, 860-866 (1937), J. B. Nierderl and I. W. Ruderman, Journal of American Chemical Society, 67, pages 1176-1177 (1945), and R. F. Hunter and V. Vand, Journal of Applied Chemistry (London), 1, page 298 (1951)]. Novolak type alkyl-substituted phenolic resins have a low number average molecular weight and a low melting point, though they have a chain-like or linear structure.
Also attempts have been made to obtain a high-molecular-weight novolak type chlorophenol resin by polycondensing p-chlorophenol or o-chlorophenol with formaldehyde in the presence of an acid catalyst. For example, W. J. Burke and S. H. Ruteman et al, Journal of Polymer Science, 20, 75-88 (1956) discloses that a high-molecular-weight novolak type chlorophenol resin having a number average molecular weight higher 1600 or 3300 is obtained by polycondensing p-chlorophenol with formaldehyde; W. J. Burke and S. H. Ruteman, Journal of Polymer Science, 32, pages 221-228 (1958) discloses that a high-molecular-weight novolak type chlorophenol resin having a number average molecular weight higher 1610 or 3640 is obtained by similarly polycondensing p-chlorophenol with formaldehyde and acetylating the resulting reaction product. However, these high-molecular-weight novolak type chlorophenol resins have been denied by researches made afterwards and it has been proved that the acetylation product is a low-molecular-weight novolak type chlorophenol resin having a number average molecular weight lower than 1250 [Minoru Imoto and Keikichi Uno, Lectures on Polymerization Reactions, Volume 8, Polyaddition and Addition Condensation (published by Kagaku Dojin) and Minoru Imoto and Shinichi Nakade, Bulletin of Chemical Society of Japan, 36, pages 580-585 (1963)]. In short, novolak type chlorophenol resins disclosed in these prior art references have a low number average molecular weight and a low melting point though they have a chain-like or linear molecular structure, and even if these novolak type chlorophenol resins are incorporated in setting resins to form resin compositions, it is impossible to sufficiently improve the heat resistance and mechanical properties as in the case of other novolak type phenolic resin described above.
Japanese Patent Application Laid-Open Specification No. 116081/79 teaches that a phenol and an aldehyde are subjected to addition condensation in the presence of an acid catalyst and a Lewis acid is added to effect removal of the phenol at an appropriate time during a period of from the point before completion of the reaction to the point after removal of condensed water, whereby a linear novolak type phenolic resin free of a three-dimensional crosslinked structure (gelation product) is obtained. In Examples of this specification, only phenol is used as the phenol, but in the text of the specification, it is taught that not only phenol but also alkyl phenols such as cresol and p-tert-butylphenol can be used as the phenol. We made the tracing experiments of Examples of this laid-open specification, and found that each of the obtained novolak type phenol-formaldehyde resin contains a considerable amount of a three-dimensional crosslinked structure (gelation product) and the number average molecular weight of the residue left after removal of the gelation product is in the range of from about 500 to about 1100 and this residue is a low-molecular-weight novolak type phenolic resin (see Comparative Example 4 given hereinafter). This is quite obvious if it is taken into account that even when dehydration condensation of a low-molecular-weight novolak type phenolic resin is conducted for a long time, if a component capable of extending the chain is not present in the reaction system, increase of the molecular weight should naturally be limited.